The present invention relates to a heat-sensitive indicator which gives an irreversible indication when the temperature of a desired subject exceeds a predetermined level and more specifically to a heat-sensitive indicator used for temperature-control in the fields of frozen foods and medical care products which require low temperature storage.
In the recent eating habits, a large number of frozen foods have frequently been used, but the frozen temperatures thereof largely differ from one another. An increase in the temperature thereof during transportation leads to quality reduction and proliferation of bacteria which, in turn, result in deterioration and rottenness. For this reason, severe conditions for temperature-control has been required for these frozen foods. In the field of medical care, the role of low temperature-control is very important for particular medicines, blood and specimens. In this case, if the temperature of these products increases, they likewise cause quality deterioration and in the worst case, they cannot be used.
As indicators for temperature-control of the foregoing foods and medical products for medical care, there have been known, for instance, time-temperature integrating monitors which undergo temperature histeresis phenomena such as those disclosed in Japanese Patent Provisional Publication Nos. 50-60262 and 51-33432; and temperature-time monitors which make use of microcapsules such as those disclosed in Japanese Patent Provisional Publication Nos. 61-13116 and 62-197486 as well as indicators which make use of capillary phenomena such as those disclosed in Japanese Patent Provisional Publication No. 59-164929.
It has been very difficult to use an indicator for low temperature-control since the temperature histeresis thereof is quite complicated. In case of the foregoing indicators for temperature-control, they are cooled prior to the use thereof and then the reaction thereof is initiated. However, most of them suffer from problems concerning the timing and mechanism for the initiation of the reaction. Moreover, substances used in these low temperature reactions have low storage stability and this often greatly affects the accuracy of the resulting indicators.